(a) Field of the Invention
The present invention relates to a hydrocarbon-based proton-conducting polymer, an electrolyte membrane including the polymer, a membrane-electrode assembly including the electrolyte membrane, and a fuel cell including the membrane-electrode assembly.
(b) Description of the Related Art
Polymer electrolyte fuel cells (PEFCs), also known as solid polymer electrolyte fuel cell (SPEFC), proton exchange membrane fuel cell (PEMFC), etc, are fuel cells that use a polymer membrane with proton exchange characteristics.
In contrast to other fuel cells, PEFCs are characterized by a low operating temperature of about 80° C., high efficiency, high current density, high output density, fast start-up time, and a rapid response to load changes. Particularly, PEFCs using a polymer membrane as an electrolyte do not require adjusting the electrolyte and not particularly sensitive to pressure changes of the reactive gas. PEFCs also feature a simple design, ease of fabrication, and a wide range of outputs; consequently, their use is well suited to a variety of applications, including power sources for zero emission vehicles, on-site generators, portable power sources, military power sources, and the like.
In PEFCs, the characteristics of the proton exchange membrane are usually described in terms of ion exchange capacity (IEC), or equivalent weight (EW). The polymer electrolyte membrane requirements for a fuel cell are high proton conductivity, high mechanical strength, and low permeability to gas and water. Resistance to dehydration is also required because the polymer electrolyte membrane of a fuel cell has a drastic drop in proton conductivity upon dehydration. The electrolyte membrane also needs to display strong resistance to the reactions (such as, e.g., oxidation/reduction reaction, hydrolysis, and the like) that directly affect the electrolyte membrane, as well as high bond strength to protons, and good homogeneity within the membrane. The electrolyte membrane should be able to maintain these properties for a defined period of time. As well as providing an electrolyte membrane meeting all these requirements, there is also a demand for developing low-cost and environmentally-friendly fabrication techniques in order to help commercialization of the electrolyte membrane.
The polymer electrolyte membranes are classified into perfluorinated electrolyte membranes, partially fluorinated electrolyte membranes, and hydrocarbon-based electrolyte membranes. The perfluorinated electrolyte membranes are commercially available as Nafion® from Dufont, Aciplex® from Asahi Chemical, Flemion® from Asahi Glass, etc. These commercially available perfluorinated electrolyte membranes suffer from several major drawbacks. For example, they have a very high cost of production, high methanol permeability, and their conductivity decreases dramatically at high temperature.
Compared with the perfluorinated electrolyte membranes, the partially fluorinated electrolyte membranes feature a lower production cost and a higher physical/chemical stability; unfortunately, they have a significantly shorter life time.
The hydrocarbon-based electrolyte membranes may include, for example, polyimide (PI), polysulfone (PSU), polyether ketone (PEK), polyarylene ether sulfone (PAES), polybenzimidazole (PBI), polyphenylene oxide (PPO), and the like. In comparison to the perfluorinated or partially fluorinated electrolyte membranes, the hydrocarbon-based electrolyte membrane features a lower production cost, and a higher thermal stability, which minimizes the drop in conductivity at a high temperature.
Introducing hydrophilic ionic groups into the hydrocarbon-based electrolyte membrane enhances conductivity, but a rise of the conductivity up to the level of the perfluorinated electrolyte membrane may cause excessive swelling of the electrolyte membrane, which results in a deterioration of the mechanical strength of the membrane. Furthermore, excessive swelling of the electrolyte membrane causes the electrolyte membrane to dissolve in water, thereby reducing the life expectancy of the membrane as a result of gradual dissolution of the electrolyte membrane. Additionally, methanol permeability increases with an increase in the water uptake.
In an attempt to solve these problems with the hydrocarbon-based electrolyte membranes, there have been proposed a variety of methods, such as the introduction of a covalent cross-linking structure to reduce the solubility of the electrolyte membranes in water and to inhibit the elution of the resin.
For example, Sumiko Matsumura et al. in McGill University of Canada proposed a synthesis method for a proton-conducting polymer that includes introducing a hexaphenyl structure at the polymer end and then conducting sulfonation of the polymer (see, e.g., Macromolecules, volume 41, pp. 281-284). This provides a proton-conducting polymer in which the molecular weight and the degree of sulfonation can be varied by controlling the length of the polymer chain and the number of chains. However, the introduction of a hexaphenyl structure at the polymer end results in a limit on the number of sulfonic acid groups available on the polymer, so the proton-conducting polymer fails to have good performances as a substitute for the commercially available products.
A variety of methods have also been proposed for compensating for these disadvantages of the hydrocarbon-based electrolyte membrane, however, no commercially viable solutions have yet emerged.